Pulse oximeters determine an oxygen saturation level of a patient's blood, or related analyte values, based on transmission/absorption characteristics of light transmitted through or reflected from the patient's tissue. In particular, pulse oximeters generally include a probe for attaching to a patient's appendage such as a finger, earlobe or nasal septum. The probe is used to transmit pulsed optical signals of at least two wavelengths, typically red and infrared, through the patient's appendage. The transmitted signals are received by a detector that provides an analog electrical output signal representative of the received optical signals. By processing the electrical signal and analyzing signal values for each of the wavelengths at different portions of a patient pulse cycle, information can be obtained regarding blood oxygen saturation.
Conventional pulse oximeters generally employ time division multiplexed (TDM) signals. As noted above, the processing of the electrical signals involves separate consideration of the portions of the signal attributable to each of the sources. Such processing generally also involves consideration of a dark current present when neither source is in an “on” state. In TDM oximeters, the sources are pulsed at different times separated by dark periods. Because the first source “on” period, second source “on” period and dark periods occur at separate times, the associated signal portions can be easily separated for processing.
The algorithms for determining blood oxygen saturation related values are normally implemented in a digital processing unit. Accordingly, one or more analog to digital (A/D) converters are generally interposed between the detector and the digital processing unit. Conventionally, the A/D converter is operative to integrate a signal over a source cycle or dark period and to generate a digital value proportional to the integrated signal. It will be appreciated that this digital value represents an aggregated quantity and does not provide information regarding a signal value at any point on a signal waveform or the shape of the signal waveform within integration periods. The resulting digital signal or signals can be correlated to corresponding intensity values by the digital processing unit, and well known algorithms can then be utilized to obtain the desired blood oxygen saturation values.